Method for forming laterally extending dielectric layer in a trench-gate FET

ABSTRACT

A field effect transistor (FET) is formed as follows. A trench is formed in a silicon region. An oxidation barrier layer is formed over a surface of the silicon region adjacent the trench and along the trench sidewalls and bottom. A protective layer is formed over the oxidation barrier layer inside and outside the trench. The protective layer is partially removed such that a portion of the oxidation barrier layer extending at least along the trench bottom becomes exposed and portions of the oxidation barrier layer extending over the surface of the silicon region adjacent the trench remain covered by remaining portions of the protective layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/166,356, filed Jun. 24, 2005, the contents of which are incorporatedherein by reference in their entirety for all purposes.

BACKGROUND

The present invention relates in general to power field effecttransistors (FETs), and in particular to trench-gate FETs with improvedcharacteristics.

FIG. 1 is a simplified cross-section view of a portion of a conventionaltrench power metal-oxide-semiconductor field-effect transistor (MOSFET).A trench 10 is lined with a gate dielectric 12, and is filled with aconductive material 15, such as polysilicon, which forms the gate of thetransistor. The trench extends from the surface of the silicon into thesubstrate down through a body region 22 and into a drain region 16. Bodyregion 22 is p-type and drain region 16 is n-type. Drain region 16 maybe electrically contacted through the substrate (not shown) of thetransistor. Source regions 14 are formed adjacent to and on oppositesides of trench 10. An active channel region 20 is thus formed in bodyregion 22 along the trench sidewalls between source regions 14 and drainregion 16.

An important parameter in a trench power MOSFET is the total gatecharge. In some applications of conventional trench power MOSFETs, suchas DC-DC converters, the lower the gate charge the better the efficiencyof the overall design. One technique in reducing the gate charge is toreduce the gate to drain capacitance by using a thick dielectric alongthe bottom of the gate trench. Conventional local oxidation of silicon(LOCOS) processes are used in forming the thick bottom dielectric. Asilicon nitride layer is commonly formed along trench sidewalls allowinga thick dielectric to be formed along the trench bottom. However, theanisotropic etch used to remove the portion of the silicon nitride layeralong the trench bottom also removes portions of the silicon nitridelayer extending over the horizontal surfaces outside the trench.

When forming the thick dielectric along trench bottom, a similarly thickdielectric is formed over the silicon mesa surfaces adjacent to thetrench. This thick dielectric over the mesa surfaces causes a number ofproblems. First, the thick dielectric over the mesa surfaces typicallyoverhangs the upper trench corners, which can cause voiding in the gatepolysilicon. Additionally, removing the thick dielectric from over themesa surfaces requires substantial etching, which can also etch the gateoxide along the upper trench sidewalls. This can lead to gate shorts andyield problems. Also, the variability in the thickness of the dielectricover the mesa surfaces causes variability in the body implant process,which in turn causes variability in the electrical parameters.

Another well-known trench-gate structure is the shielded gate structure.The trench in this structure includes a shield electrode directly belowthe gate electrode. The shield electrode is insulated from adjacentsilicon regions by a shield dielectric which is generally thicker thanthe gate dielectric. The gate and shield electrodes are insulated fromone another by a dielectric layer commonly referred to as inter-polydielectric or IPD. The IPD is required to be of high quality and towithstand the potential difference between the shield and gateelectrodes. One approach in forming the IPD may be to thermally oxidizethe shield electrode during the gate oxidation process. However, thisapproach limits the thickness of the IPD because it is formed at thesame time as the gate oxide. Also, the quality of oxide grown onpolysilicon is not as good as oxide grown on single crystal silicon.Therefore, it is desirable that the dielectric on the shield polysilicon(the IPD) be much thicker than the gate dielectric so that the IPD canwithstand at least as much voltage as the gate dielectric is capable of.

Thus, there is a need for improved techniques for forming the thickdielectric along the trench bottom, and for forming the IPD in shieldedgate structures.

SUMMARY

In accordance with an embodiment of the invention, a field effecttransistor (FET) is formed as follows. A trench is formed in a siliconregion. An oxidation barrier layer is formed over a surface of thesilicon region adjacent the trench and along the trench sidewalls andbottom. A protective layer is formed over the oxidation barrier layerinside and outside the trench. The protective layer is partially removedsuch that a portion of the oxidation barrier layer extending at leastalong the trench bottom becomes exposed and portions of the oxidationbarrier layer extending over the surface of the silicon region adjacentthe trench remain covered by remaining portions of the protective layer.

In one embodiment, the exposed portion of the oxidation barrier layer isremoved from along the trench bottom, and a dielectric layer is formedalong the bottom of the trench.

In another embodiment, prior to forming the oxidation barrier layer, aninsulating layer is formed over the surface of the silicon regionadjacent to the trench and along the trench sidewalls and bottom,wherein removing the exposed portion of the oxidation barrier layerexposes a portion of the insulating layer extending along the trenchbottom.

In accordance with another embodiment of the invention, an intermediaryof a FET includes a trench extending into a silicon region, and anoxidation barrier layer which extends over a surface of the siliconregion adjacent the trench and along the trench sidewalls but isdiscontinuous along the trench bottom. A protective layer extends overall horizontally extending portions of the oxidation barrier layer.

In one embodiment, an insulating layer extends over the surface of thesilicon region adjacent to the trench and along the trench sidewalls andbottom. The insulating layer is covered by the oxidation barrier layerexcept for the portion of the insulating layer extending along thetrench bottom.

In another embodiment, the oxidation barrier layer has a substantiallyuniform thickness and comprises silicon nitride, and the protectivelayer comprises low temperature oxide (LTO).

The following detailed description and the accompanying drawings providea better understanding of the nature and advantages of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified cross-section view of a typical trench-gateMOSFET;

FIGS. 2A-2J show simplified cross-section views at various steps in amanufacturing process for forming a trench-gate FET with an improvedthick bottom dielectric (TBD), in accordance with an embodiment of theinvention; and

FIGS. 3A-3J show simplified cross-section views at various steps in amanufacturing process for forming a shielded gate trench FET with animproved inter-poly dielectric (IPD), in accordance with anotherembodiment of the invention.

DETAILED DESCRIPTION

In accordance with an embodiment of the invention, a thick dielectriclayer is formed along the bottom of a trench in a trench-gate FET whilepreventing formation of a similarly thick dielectric layer over thesilicon mesa surfaces adjacent the trench by using a protective layer.The protective layer serves to protect portions of a silicon nitridelayer extending over the silicon mesa surfaces during a silicon nitrideetch process for removing the silicon nitride layer from along thetrench bottom. In accordance with another embodiment of the invention, asimilar technique is advantageously used to form the inter-polydielectric in a shielded gate trench FET.

FIGS. 2A-2J show cross-section views at various steps in a manufacturingprocess for forming a trench-gate FET with a thick bottom dielectric, inaccordance with an embodiment of the invention. The followingdescription of the steps in the process flow is only exemplary and itshould be understood that the scope of the invention is not to belimited to these particular examples. In particular, processingconditions such as temperature, pressure, layer thickness, and the likecould be varied, without departing from the spirit of the invention.

In FIG. 2A, a silicon region 30 is provided as the basis for forming thetransistor. In one embodiment, silicon region 30 is an n-type epitaxiallayer formed over a highly doped n-type substrate (not shown) formingthe transistor drain region. A conventional anisotropic silicon etch isperformed to form a trench 32 extending into silicon region 30. Anoptional anneal process may be performed to round the trench corners andto reduce the defect density of silicon region 30.

In FIG. 2B, a dielectric layer 40 is formed covering the trench bottomand sidewalls as well as surface regions 38 of the silicon mesa adjacentthe trench. In one embodiment, dielectric layer 40 is a pad oxide havinga thickness in the range of 200-1,000 Å using conventional techniques.

In FIG. 2C, an oxidation barrier layer 42 is formed over dielectriclayer 40. The oxidation barrier layer 42 has a substantially uniformthickness. In one embodiment, layer 40 is from pad oxide, and layer 42is from silicon nitride formed using a low pressure chemical vapordeposition (LPCVD) process. The LPCVD process helps achieve asubstantially uniform silicon nitride layer. The pad oxide improves theadhesion of the silicon nitride layer, and also serves to protect theunderlying silicon along the trench bottom during a nitride etch carriedout in later steps corresponding to FIG. 2F. Oxidation inhibitingmaterial other than nitride may also be used, and the precisecharacteristics of the oxidation barrier layer 42 may be varied bychanges to gas ratios, temperature, pressure, and spacing of componentsin a CVD chamber.

In FIG. 2D, a protective layer 44 is formed over the oxidation barrierlayer 42. A low-temperature oxide (LTO) film may be used as protectivelayer 44. However, other materials exhibiting similar characteristicsmay also be used. The LTO film is deposited using a standard chemicalvapor deposition (CVD) process. This process yields a non-uniform filmwhich is thickest over mesa surfaces 38 and gradually thins down alongthe trench sidewalls to a smaller thickness along the lower portion oftrench sidewalls and along the trench bottom.

In FIG. 2E, protective layer 44 is uniformly etched back, using forexample a timed wet etch process, such that a portion of oxidationbarrier layer 42 extending along the trench bottom and the lowersidewalls of the trench becomes exposed while those portions ofoxidation barrier layer 42 extending over silicon mesa surface regions38 remain covered by remaining portions of protective layer 44. In oneembodiment, a 50:1 HF (hydrofluoric) buffered oxide etch is used as theetching process, however other etch solutions exhibiting similarproperties may also be used.

In FIG. 2F, a portion of oxidation barrier layer 42 is removed from thetrench bottom. In the embodiment where oxidation barrier layer 42 isfrom silicon nitride, the portion of the silicon nitride layer along thetrench bottom is removed using a highly directional anisotropic etchprocess. This exposes the pad oxide layer along the trench bottom. Thepresence of protective layer 44 prevents the etch process from removingthe oxidation barrier layer 42 from over the silicon mesa surfaces 38.Pad oxide layer 40 prevents the nitride etch from attacking theunderlying silicon along the trench bottom. In alternative embodiments,a variety of etchant materials and etch environments may be used. Etchvariables such as temperature, pressure and RF power of the etchingprocess may be tuned to accomplish the desired anisotropic etch process.

In FIG. 2G, another wet etch process is used to remove the remainingportions of protective layer 44. This will also remove the exposedportion of pad oxide layer 40 along the trench bottom although suchremoval is not necessary. A surface area of silicon region 30 along thetrench bottom is thus exposed. In an exemplary embodiment, a 50:1 HFbuffered oxide etch is used as the etching process, however other etchsolutions exhibiting similar properties may also be used.

In FIG. 2H, a thick layer of oxide 48 is formed along the exposedsilicon surface area along the trench bottom using conventional localoxidation of silicon (LOCOS) process. The oxidation step consumes aportion of the exposed silicon region. Oxidation barrier layer 42inhibits oxidation of the silicon along trench sidewalls and the topsurface regions 38 during the LOCOS process. In one embodiment, thethick bottom dielectric (TBD) layer 48 is formed using an oxidationprocess in the temperature range of, for example, 800-1250° C. Theprocess can be either a wet or dry oxidation, depending on thecapability of the oxidation barrier layer 42. The desired thickness ofTBD layer 48 can be obtained by altering the parameters of the oxidationprocess. In one embodiment, in the steps corresponding to FIG. 2G, theremaining portions of protective layer 44 are selectively removed suchthat pad oxide layer 40 along the trench bottom remains intact. Thisresults in formation of an even thicker TBD.

In FIG. 2I, a pair of conventional etch processes are carried out toremove oxidation barrier layer 42 and pad oxide 40, leaving only the TBDlayer 48 along the trench bottom. In an exemplary embodiment, aconventional silicon nitride etch process is used to remove siliconnitride layer 42, and a conventional oxide etch process is used toremove pad oxide layer 40. The oxide etch process also removes a thinlayer off the top of TBD layer 48, however this amount may becompensated for by modifying the oxide growth recipe parameters duringthe formation of TBD layer 48.

With the formation of the TBD layer, the remaining portions of thetrench FET structure can be formed using any one of a number of knowntechniques, one of which is briefly described next. In FIG. 2J, a gatedielectric layer 50 is formed along the trench sidewalls. A recessedgate electrode 52, e.g., from polysilicon, is then formed in the trench.Body regions 56 of p-type conductivity and source regions 54 of n-typeconductivity are formed in silicon region 30 using conventional ionimplantation techniques. Alternatively, source regions 54 and bodyregions 56 can be formed prior to etching the trench in silicon region30.

The cross section in FIG. 2J corresponds to an embodiment where an opencell configuration is used with source regions 54 and trenches 32 beingstripe-shaped extending parallel to one another. In this embodiment,conventional techniques are used to form heavy body regions of p-typeconductivity periodically along the source stripes. The heavy bodyregions extend through source regions 54 and terminate in body regions56. A dielectric layer, e.g., BPSG, is formed over the structure andpatterned, and a reflow process results in formation of a dielectricdome 58 extending over the trench and portions of source regions 54. Atop metal layer 60 for electrically contacting source regions 54 and theheavy body regions is formed over the entire structure. The structureand method of the present invention are not limited to open cellconfiguration. The implementation of the present invention in a closedcell configuration would be obvious to one skilled in this art in viewof this disclosure.

As can be seen in FIG. 2F, protective layer 44 prevents thehorizontally-extending portions of silicon nitride layer 42 over thesilicon mesa surfaces from being removed, which in turn prevents theformation of the thick dielectric over the silicon mesa surfaces. Thisis advantageous for a number of reasons. First, the absence of a thickdielectric over the mesa surfaces greatly reduces the chance ofpolysilicon voiding, thus improving the manufacturing yield. Second,there is no need to remove the thick dielectric from over the siliconmesa surfaces after etching back the polysilicon layer, thus reducingthe number of processing steps and eliminating the possibility ofremoving portions of the gate dielectric from along the upper trenchsidewalls. Third, the absence of the thick dielectric over the mesasurfaces reduces the variability in the body implant process, thusallowing for better controlled implant characteristics as well asreducing variations in the electrical parameters of the transistor.Finally, the TBD is formed in a self-aligned manner. That is, no maskinglayer is used in the process sequence leading to formation of the TBD.

The process steps for forming the TBD described above may beadvantageously integrated with the process described in the commonlyassigned patent application Ser. No. 10/442,670, titled “Structure andMethod for Forming a Trench MOSFET Having Self-Aligned Features”, filedMay 20, 2003, incorporated herein by reference in its entirety, toobtain a FET with superior characteristics.

A similar technique to that for forming the TBD layer 48 may beadvantageously used to form the inter-poly dielectric (IPD) layer in ashielded gate FET. FIGS. 3A-3J show cross-section views at various stepsin a manufacturing process for forming such a shielded gate FET, inaccordance with another embodiment of the invention.

In FIG. 3A, a trench 132 is formed in silicon region 130, and a shielddielectric layer is formed along the trench sidewalls and bottom andover the silicon surface 138 adjacent the trench using conventionaltechniques. A shield electrode 104, e.g., from polysilicon, is thenformed in the lower portion of the trench over the shield dielectriclayer using known techniques. A wet etch step is then carried out toremove the exposed portions of the shield dielectric layer, i.e., theportions over silicon surface 138 and along upper portions of trenchsidewalls, thus leaving behind recessed shield dielectric 102. Next, theIPD is formed using a similar technique to that for forming the TBD inthe FIGS. 2A-2J embodiment.

In FIG. 3B, a dielectric layer 140 is formed over the structure,covering the silicon surface 138 and extending along the trenchsidewalls and over shield electrode 104. In one embodiment, dielectriclayer 140 is a thermally grown pad oxide layer. In FIG. 3C, an oxidationbarrier layer 142 is then formed over dielectric layer 140. Siliconnitride may be used as the oxidation barrier layer, however, othermaterials exhibiting similar oxidation inhibiting properties may also beused. The LPCVD process may be used to form a substantially uniformsilicon nitride layer.

In FIG. 3D, a protective layer 144 is formed over oxidation barrierlayer 142. A low-temperature oxide (LTO) film may be used as protectivelayer 144 however other materials exhibiting similar characteristics mayalso be used. The LTO film is deposited using a standard chemical vapordeposition (CVD) process. This process yields a non-uniform film whichis thickest over mesa surfaces 138 and gradually thins down along thetrench sidewalls to a smaller thickness along the lower portion oftrench sidewalls and over shield electrode 104.

In FIG. 3E, protective layer 144 is uniformly etched back, using forexample a timed wet etch process, such that a portion of oxidationbarrier layer 142 extending over shield electrode 104 and along thelower sidewalls of the trench becomes exposed while those portions ofoxidation barrier layer 142 extending over silicon mesa surface regions138 remain covered by remaining portions of protective layer 144.

In FIG. 3F, a portion of oxidation barrier layer 142 is removed fromover shield electrode 104. In the embodiment where layer 142 is fromsilicon nitride, the portion of silicon nitride layer extending over theshield electrode is removed using a highly directional anisotropic etchprocess. This exposes the portion of pad oxide layer 140 extending overthe shield electrode. The presence of protective layer 144 prevents theetch process from removing the oxidation barrier layer 142 from oversilicon mesa surfaces 138. Pad oxide layer 140 prevents the nitride etchfrom attacking the underlying polysilicon shield electrode.

In FIG. 3G, another wet etch process is used to remove the remainingportions of protective layer 144. This will also remove the exposedportion of pad oxide layer 140 along the trench bottom although suchremoval is not necessary. A top surface of shield electrode 104 is thusexposed. In FIG. 3H, a thick layer of inter-poly dielectric (IPD) 148 isformed along the exposed top surface of shield electrode 104 usingconventional local oxidation of silicon (LOCOS) process. The oxidationstep consumes a thin layer of shield electrode 104. Oxidation barrierlayer 142 inhibits oxidation of the silicon along trench sidewalls andthe top silicon surface regions 138 during the LOCOS process. Thedesired thickness of IPD layer 148 can be obtained by altering theparameters of the oxidation process. In one embodiment, in the stepscorresponding to FIG. 3G, the remaining portions of protective layer 144are selectively removed such that pad oxide layer 140 along the trenchbottom remains intact. This results in formation of an even thicker IPD.

In FIG. 3I, a pair of conventional etch processes are carried out toremove oxidation barrier layer 142 and pad oxide 140, leaving only IPDlayer 148 over shield electrode 104. In an exemplary embodiment, aconventional silicon nitride etch process is used to remove siliconnitride layer 142, and a conventional oxide etch process is used toremove pad oxide layer 140. The oxide etch process also removes a thinlayer off the top of IPD layer 148, however this amount may becompensated for by modifying the oxide growth recipe parameters duringthe formation of IPD layer 148.

The remaining portions of the trench FET structure can be formed usingany one of a number of known techniques, one of which is brieflydescribed next. In FIG. 3J, a gate dielectric layer 150 is formed alongthe trench sidewalls. A recessed gate electrode 152, e.g., frompolysilicon, is then formed in the trench. Body regions 156 of p-typeconductivity and source regions 154 of n-type conductivity are formed insilicon region 130 using conventional ion implantation techniques.Alternatively, source regions 154 and body regions 156 can be formedprior to etching the trench in the silicon or at some other process stepprior to forming the IPD. A dielectric layer, e.g., BPSG, is formed overthe structure and patterned, and a reflow process results in formationof a dielectric dome 158 extending over the trench and portions ofsource regions 154. A top metal layer 160 is formed over the entirestructure. Heavy body regions (not shown) of p-type conductivity areformed extending through source regions 154 and terminating in bodyregions 156 using conventional techniques. As with the previousembodiment, the structure and method of the present embodiment may beimplemented in either an open cell configuration or a closed cellconfiguration.

The various structures and methods of the present invention may becombined with one or more of a number of charge spreading techniques aswell as other device structures and manufacturing processes disclosed inthe commonly assigned application Ser. No. 11/026,276, filed Dec. 29,2004, and incorporated herein by reference in its entirety, to achieve alower on-resistance, higher blocking capability and higher efficiency,among other advantages and features.

The cross-section views of the different embodiments may not be toscale, and as such are not intended to limit the possible variations inthe layout design of the corresponding structures. Also, the varioustransistors can be formed in stripe or cellular architecture includinghexagonal or square shaped transistor cells.

Although a number of specific embodiments are shown and described above,embodiments of the invention are not limited thereto. For example, it isunderstood that the doping polarities of the structures shown anddescribed could be reversed and/or the doping concentrations of thevarious elements could be altered without departing from the invention.As another example, the transistors described above have the trenchesterminating before reaching the more heavily doped substrate, but theycan also extend into and terminate within the substrate. Also, while thevarious embodiments described above are implemented in conventionalsilicon, these embodiments and their obvious variants can also beimplemented in silicon carbide, gallium arsenide, gallium nitride,diamond or other semiconductor materials. Further, the features of oneor more embodiments of the invention may be combined with one or morefeatures of other embodiments of the invention without departing fromthe scope of the invention.

Therefore, the scope of the present invention should be determined notwith reference to the above description but should, instead, bedetermined with reference to the appended claim, along with their fullscope of equivalents.

1. A method of forming a FET, comprising: forming a trench in a siliconregion; forming a silicon nitride layer over a surface of the siliconregion adjacent the trench and along the trench sidewalls and bottom;forming an oxide layer over the silicon nitride layer such that theoxide layer is thicker along the surface of the silicon region adjacentthe trench than along the trench bottom; and uniformly etching back theoxide layer such that a portion of the silicon nitride layer extendingalong the trench bottom and along at least a portion of the trenchsidewalls becomes exposed while portions of the silicon nitride layerextending over the surface of the silicon region adjacent the trenchremain covered by remaining portions of the oxide layer.
 2. The methodof claim 1 further comprising: removing the exposed portion of thesilicon nitride layer along the trench bottom using an anisotropic etchprocess; and forming a dielectric layer along the trench bottom using alocal oxidation of silicon (LOCOS) process.
 3. The method of claim 1wherein the silicon nitride layer is formed using a low pressurechemical vapor deposition (LPCVD) process.
 4. The method of claim 2further comprising: prior to forming the silicon nitride layer, forminga nitride etch resistant layer to protect the silicon region along thetrench bottom during the removing step.
 5. The method of claim 4 whereinthe nitride etch resistant layer comprises pad oxide, and extends overthe surface of the silicon region adjacent to the trench and along thetrench sidewalls and bottom, wherein the step of removing the exposedportion of the silicon nitride layer exposes a portion of the pad oxidelayer extending along the trench bottom.
 6. A method of forming a FET,comprising: forming a trench in a semiconductor region; forming anoxidation barrier layer over a surface of the semiconductor regionadjacent the trench and along the trench sidewalls and bottom; forming aprotective layer over the oxidation barrier layer inside and outside thetrench; partially removing the protective layer such that a portion ofthe oxidation barrier layer extending at least along the trench bottombecomes exposed and portions of the oxidation barrier layer extendingover the surface of the semiconductor region adjacent the trench remaincovered by remaining portions of the protective layer; removing theexposed portion of the oxidation barrier layer from along the trenchbottom; and thereafter forming a dielectric layer along the bottom ofthe trench.
 7. The method of claim 6 further comprising: prior toforming the oxidation barrier layer, forming an insulating layer overthe surface of the semiconductor region adjacent to the trench and alongthe trench sidewalls and bottom, wherein removing the exposed portion ofthe oxidation barrier layer exposes a portion of the insulating layerextending along the trench bottom.
 8. The method of claim 6 wherein theoxidation barrier layer comprises silicon nitride, and the step ofremoving the exposed portion of the oxidation barrier layer is carriedout using an anisotropic etch process.
 9. The method of claim 6 whereinthe silicon nitride layer is formed using a low pressure chemical vapordeposition (LPCVD) process.
 10. The method of claim 6 wherein thedielectric layer is formed using a local oxidation of silicon (LOCOS)process.
 11. The method of claim 6 wherein the protective layercomprises oxide.
 12. The method of claim 6 wherein the portion of theprotective layer extending along the surface of the semiconductor regionadjacent the trench is thicker than the portion of the protective layerextending along the trench bottom.
 13. The method of claim 12 whereinthe step of partially removing the protective layer comprises: uniformlyetching the protective layer.